The goal of plant breeding is to combine in a single variety or hybrid various desirable traits of the parental lines. For field crops, these traits may include resistance to diseases and insects, tolerance to heat and drought, reduced time to crop maturity, greater yield, and better overall agronomic quality. With mechanical harvesting of many crops, uniformity of plant characteristics such as germination, stand establishment, growth rate, maturity, and fruit size, is important. Plant breeding begin with the analysis and definition of problems and weaknesses of the current germplasm, the establishment of program goals and the definition of specific breeding objectives. The next step is selection of germplasm that possesses the traits to meet the program goals, and breeding to gain reproducible expression of that trait.
Field crops are bred through techniques that take advantage of the plant's method of pollination. A plant is self-pollinating if pollen from one flower is transferred to the same or another flower of the same plant. A plant is cross-pollinated if the pollen comes from a flower on a different plant. In self-pollinating species, such as soybeans and cotton, the male and female organs are anatomically juxtaposed. During natural pollination, the male reproductive organs of a given flower pollinate the female reproductive organs of the same flower. Maize can self or cross pollinate. In Brassica, the plant is normally self sterile and can only be cross-pollinated.
Soybean plants are taxonomically classified in the genus Glycine, which contains two subgenera, Glycine and Soja. Under the subgenus Soja are two species; Glycine max, a cultivated species, and Glycine soja, a wild species.
Soybean plants are recognized to be naturally self-pollinated plants which, while capable of undergoing cross-pollination, do so infrequently in nature. Insects are reported by some researchers to carry pollen from one soybean plant to another and it generally is estimated that less than one percent of soybean seed formed in an open planting can be traced to cross-pollination, i.e. less than one percent of soybean seeds formed in an open planting is capable of producing F1 hybrid soybean plants, See Jaycox, "Ecological Relationships between Honey Bees. and Soybeans," appearing in the American Bee Journal Vol. 110(8): 306-307 (August 1970). This reference and all references cited are incorporated herein by reference. Thus intervention for control of pollination is critical to establishment of superior varieties.
One of the most critical aspects of plant breeding is the ability to control the pollination process so that parental lines with desired traits are intentionally crossed to provide those same traits in the offspring.
Large scale commercial hybrid production, as it is practiced today, requires the use of some form of male sterility system which controls or inactivates male fertility. A reliable method of controlling male fertility in plants also offers the opportunity for improved plant breeding. This is especially true for development of maize hybrids, which relies upon some sort of male sterility system. There are several options for controlling male fertility available to breeders, such as: manual or mechanical emasculation (or detasseling for maize), cytoplasmic male sterility, genetic male sterility, gametocides and the like. Most advances in male sterility have occurred with maize production.
Hybrid maize seed is typically produced by a male sterility system incorporating manual or mechanical detasseling. Alternate strips of two inbred varieties of maize are planted in a field, and the pollen-bearing tassels are removed from one of the inbreds (female) prior to pollen shed. Providing that there is sufficient isolation from sources of foreign maize pollen, the ears of the detasseled inbred will be fertilized only from the other inbred (male), and the resulting seed is therefore hybrid and will form hybrid plants.
The laborious, and occasionally unreliable, detasseling process can be avoided by using cytoplasmic male-sterile (CMS) inbreds. Plants of a CMS inbred are male sterile as a result of factors resulting from the cytoplasmic, as opposed o the nuclear, genome. Thus, this characteristic is inherited exclusively through the female parent in maize plants, since only the female provides cytoplasm to the fertilized seed. CMS plants are fertilized with pollen from another inbred that is not male-sterile. Pollen from the second inbred may or may not contribute genes that make the hybrid plants male-fertile. Usually seed from detasseled fertile maize and CMS produced seed of the same hybrid are blended to insure that adequate pollen loads are available for fertilization when the hybrid plants are grown.
There are many other methods of conferring genetic male sterility in the art, each with its own benefits and drawbacks. These methods use a variety of approaches such as delivering into the plant a gene encoding a cytotoxic substance associated with a male tissue specific promoter or an antisense system in which a gene critical to fertility is identified and an antisense to that gene is inserted in the plant (see: Fabinjanski, et al. EPO 89/3010153.8 Publication No. 329,308 and PCT application PCT/CA90/00037 published as WO 90/08828).
Another system useful in controlling male fertility makes use of gametocides. Gametocides are not a genetic system, but rather a topical application of chemicals. These chemicals affect cells that are critical to male fertility. The application of these chemicals affects fertility in the plants only for the growing season in which the gametocide is applied (see Carlson, Glenn R., U.S. Pat. No. 4,936,904). Application of the gametocide, timing of the application and genotype specificity often limit the usefulness of the approach.
Male sterility is a general phenomenon in the plant kingdom. Duvick (1966) suggested that all plant species must have at least one nuclear gene for male sterility. Laser et al. (1972) noted that there were published reports of cytoplasmic male sterility (CMS) in approximately 140 species. The number of species in which CMS has been found had greatly increased by the time of Kaul's (1988) review.
Several types of male sterile soybeans have been identified (Palmer, et al. 1987). For example, genetic male sterility (GMS), controlled by single recessive gene, has been intensively investigated and reviewed (Graybosch et al. 1988, Palmer et al. 1992). Those materials have been used in genetic studies and breeding programs such as recurrent selection.
Because of the difficulty in obtaining pure strands of male sterile plants and because of the difficulty in achieving cost-effective pollination, the use of GMS in hybrid soybean seed production is not widely practiced at present. However, the CMS system has proven to be efficient in hybrid seed production in several important crops, including maize, sorghum and rice. Cytoplasmic male sterility and restorer genes have been reported in soybeans. Davis (U.S. Pat. No. 4,545,146; U.S. Pat. No. 4,763,441, and U.S. Pat. No. 4,648,204) has disclosed cultivars which reportedly contribute male sterile cytoplasm and two pairs of recessive genes r1r1, r2r2 for male sterile maintenance. This CMS system is best described as a two gene, sporophytic system in which varieties possessing at least one allele that is dominant in each fertility restorer gene pair, R1R1 or R2R2, lead to viable pollen production even in the presence of CMS cytoplasm. There has been no reported independent verification of this system. For example, given the available seed stocks, sterility is not expressed even when the required atypical CMS cytoplasm and recessive restorer genes are present. See Davis U.S. Pat. No. 4,545,146 column 7, line 30 through column 9, line 63, where the sporophytic two- gene system is discussed.
With a sporophytic system of restoration in cytoplasmic male sterile lines, it is the genotype of the plant (sporophyte) that determines whether normal pollen is produced. Thus, a sporophytic plant that is heterozygous at a restorer gene locus produces all normal pollen even though only one-fourth of the pollen grains carry both dominant alleles. In a gametophytic system it is the genotype of the pollen grain (gametophyte) itself treat determines whether the pollen grain is normal or abnormal. Therefore such a plant that is heterozygous for a restorer gene produces one-half normal and one-half aborted pollen grains depending upon whether the dominant allele or the recessive allele is present. This is often referred to as "semi-sterility".
The invention described herein is a gametophytic, cytoplasmic male sterile soybean line. Further, unlike Davis, it requires manipulation of only one restorer gene and not two, making it easier to use in breeding.